Kinematically aligned total knee arthroplasty limits high tibial forces, differences in tibial forces between compartments, and abnormal tibial contact kinematics during passive flexion

Abstract

Purpose

Following total knee arthroplasty (TKA), high tibial forces, large differences in tibial forces between the medial and lateral compartments, and anterior translation of the contact locations of the femoral component on the tibial component during passive flexion indicate abnormal knee function. Because the goal of kinematically aligned TKA is to restore native knee function without soft tissue release, the objectives were to determine how well kinematically aligned TKA limits high tibial forces, differences in tibial forces between compartments, and anterior translation of the contact locations of the femoral component on the tibial component during passive flexion.

Methods

Using cruciate retaining components, kinematically aligned TKA was performed on thirteen human cadaveric knee specimens with use of manual instruments without soft tissue release. The tibial forces and tibial contact locations were measured in both the medial and lateral compartments from 0° to 120° of passive flexion using a custom tibial force sensor.

Results

The average total tibial force (i.e. sum of medial + lateral) ranged from 5 to 116 N. The only significant average differences in tibial force between compartments occurred at 0° of flexion (29 N, p = 0.0008). The contact locations in both compartments translated posteriorly in all thirteen kinematically aligned TKAs by an average of 14 mm (p < 0.0001) and 18 mm (p < 0.0001) in the medial and lateral compartments, respectively, from 0° to 120° of flexion.

Conclusions

After kinematically aligned TKA, average total tibial forces due to the soft tissue restraints were limited to 116 N, average differences in tibial forces between compartments were limited to 29 N, and a net posterior translation of the tibial contact locations was observed in all kinematically aligned TKAs during passive flexion from 0° to 120°, which are similar to what has been measured previously in native knees. While confirmation in vivo is warranted, these findings give surgeons who perform kinematically aligned TKA confidence that the alignment method and surgical technique limit high tibial forces, differences in tibial forces between compartments, and anterior translation of the tibial contact locations during passive flexion.

Appendices

Appendix 1: Contribution of applied muscle loads to differences in tibial forces between compartments

The differences in tibial forces between compartments (F _diff,i ) created by the applied muscle loads at each flexion angle (i = 0°, 30°, 60°, 90°, and 120°) can be estimated using average anatomy and lines of action [3, 11] (Fig. 10, Eq. 1) where $\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {r}_{\text{BF}}$, $\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {r}_{\text{SMST}}$, and $\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {r}_{\text{Q}}$ are the vectors from the centre of the tibial plateau to the tibial insertions of the biceps femoris (BF), semitendinosus/semimembranosus (SMST), and the patellar tendon (Q), respectively; $\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{BF}},i}}$, $\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{SMST}},i}}$, and $\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{Q}},i}}$ are the vectors whose orientation is set by the line of action and magnitude is set by the applied loads at the tibial insertions of the biceps femoris, semitendinosus/semimembranosus, and the patellar tendons, respectively; w is the average medial–lateral spacing between the contact locations in the medial and lateral compartments. The magnitude of $\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{Q}},i}}$ was computed using the average ratio of the load in the quadriceps tendon to that in the patellar tendon [42].

$$ \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{diff}},i}} = \frac{{\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {r}_{\text{BF}} \times \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{BF}},i}} + \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {r}_{\text{SMST}} \times \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{SMST}},i}} + \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {r}_{\text{Q}} \times \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{Q}},i}} }}{w} $$

(1)

Appendix 2: Anterior–posterior and compression–distraction components of applied muscle loads

The total tibial force (F _total,i ) can be decomposed into the contribution of the muscle loads (F _muscle,i ) and the contribution of the soft tissue restraints ($F_{{{\text{soft}}\;{\text{tissue}},i}}$) at each flexion angle (i = 0°, 10°, 30°, 45°, 60°, 90°, and 120°) (Eq. 2). The total tibial force is calculated as the sum of the medial and lateral tibial forces computed using the tibial force sensor. The anterior–posterior (A–P) and compression–distraction (C–D) components of the muscle loads are computed based on applied load to each muscle, the alignment of the muscle loads relative to the tibia [3, 11], and the ratio of the load in the quadriceps tendon to that in the patellar tendon (r _Q/Pat,i ) [42] (Fig. 12).

$$ \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{total}},i}} = \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{muscle}},i}} + \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {F}_{{{\text{soft}}\;{\text{tissue}},i}} $$

(2)

Between 0° and 30° of flexion, the net A–P force component of the muscle loads is directed anteriorly, whereas between 30° and 120° of flexion, the net A–P force component of the muscle loads is directed posteriorly (Fig. 13). The anterior translation of both tibial contact locations between 30° and 90° (Fig. 8; Table 2) is likely driven by the muscle forces pulling the tibia posteriorly on the femur. Because the soft tissues are minimally loaded between these flexion angles (Fig. 14), the small A–P forces applied by the muscles can drive the A–P translation of the tibial contact locations.